1. Field of the Invention
The present invention relates to a tone display method of a TV image signal and more particularly to a tone display method for displaying the tone of brightness of a luminant element by changing the luminant time width by dividing the inside of the field of a TV signal into several subfields corresponding to the pixel display time and controlling light emission of the subfields and an apparatus therefor.
2. Description of the Prior Art
As a method for displaying the tone of a TV image signal by controlling the brightness of a display element, a method for controlling the luminant time width of a luminant element is conventionally known.
For example, a memory type plasma display is described in T. Kaji et al., "A Proposal of the Drive Method for TV Using AC Type Plasma Display Panel", Image Engineers Report No. IT72-45, Mar. 12, 1973, The Institute of Electronics and Communication Engineers of Japan. As shown in FIG. 2, this is a method for displaying the tone of brightness by dividing the time width of a field of a TV signal into 8 subfields corresponding to the pixel display time, weighting the time width of each of the 8 subfields in binary, and controlling the presence or absence of light emission of each subfield (b0 to b7 are named). In this case, each subfield shown in FIG. 2 is a time width coded in binary. However, as shown in FIGS. 3(a) and 3(b), for example, it is possible that the luminant time width in the subfields is not almost the entire of the period of the subfields (90% duty ratio in FIG. 3(a)) but as shown in FIG. 3(b) for example, the luminant time width is a half of the time width of the subfields (50% duty ratio).
A TV display example by this intra-field divided subfield system is described in H. Murakami et al., "A Color TV Display Using 8-Inch Pulse Discharge Panel with Internal Memory", Journal of the Institute of Television Engineers of Japan, Vol. 38, No. 9, 1984, pp. 836-842. As shown in FIG. 4, this is display of a TV image signal by dividing the period of a field of a TV signal into 8 subfields at even intervals, weighting the luminant time width of each of the subfields in binary, and controlling the presence or absence of light emission of these subfields.
According to the aforementioned prior art, it is known that when a TV image signal is displayed actually, dynamic false contour noise is generated for a moving image. For example, in T. Masuda et al., "New Category Contour Noise Observed in Pulse-Width-Modulated Moving Images", Technical Report of IEICE, Vol. 94, No. 438, EID94-126, January 1995, pp. 61-66, The Institute of Electronics, Information and Communication Engineers of Japan, when in particular, the cheek of the face and skin of a person move by a smooth tone change in the conventional tone display method, contour string noise is generated. It is described that the principle is that the luminant time pattern in several subfields in the field is converted to a spatial pattern on the retina of each eye as the viewing point of an observer moves.
As a method for reducing such dynamic false contour noise for a moving image, a method for displaying by dividing and separating some of the upper bits in a plurality of subfields is disclosed in Japanese Laid-Open Patent Application No. 4-211294 which is a publication of Japanese Patent Application No. 3-30648. However, according to this method, there is a problem imposed that the reduction of dynamic false contour noise is not sufficient and no noticeable improvement effect is produced for a rapidly moving image.